Photographic sound recording apparatus



A ril 8, 1941. E. HACK 2.237.904

PHOTOGRAPHIC .SOUND RECORDING APPARATUS Filed July 31, 1 940 2 Sheets-Sheet 1 lulllilllilllll 7' l {M/LL mEco/v s) TIME -b- (/4112. IJECOMDJ) TIME D Fia (MILL/SECONDS) F 81! Inventor Ernsz; Hash;

April 8, 1941. E. HAC 2.237.904

PHOTOGRAPHIC SOUND RECORDING APPARATUS Filed July 31, 1940 2 Sheets-sheaf 2 1 26. 4,

3maentor ma H0671? Patented Apr. 8, 1941 PHOTOGRAPHIG SOUND RECORDING APPARATUS Ernst Hack, Berlin, Germany, assignor to Klangfilm G. in. b. H., Berlin, Germany, a corporation of Germany Application July 31, 1940, Serial No. 348,859 In Germany July 10, 1939 6 Claims.

This invention relates to sound recording apparatus and more particularly to an improved amplifier for operating the ground noise reduction shutter or equivalent ground noise reduction means in a sound film recording apparatus.

this kind, for instance, is the biasing stop gate or shutter which is traversed by the noise reducing or biasing current, that is to say, a current which follows the envelope of the audio-frequencies or tonal frequencies, and is controlled thereby such that current is usually called the envelope current.

It is known that such gates or biasing shutters require more or less time to build up their motion. The result is that in the course of the opening process portions of the sound track are cut oif, and this leads to distortions. Now, this invention discloses a method whereby thesaid building-up period may be shortened. According to the invention this is insured by feeding the gate or stop not only with the normal gate current, but also with a brief surge or rush of current at the very beginning of the opening. According to the invention, the gate, after the first current rush has been delivered, may be impressed with a counteracting rush of current so as to brake or retard the motion of the gate.

One object of the invention is to provide an improved ground noise reduction amplifier.

Another object of the invention is to provide a ground noise reduction amplifier which will overcome the inertia of the ground noise reduction shutter.

Another object of the invention is to provide such an amplifier which will provide a brief surge of current at the beginning of the opening.

Another object of the invention is to provide such an amplifier which will provide a brief rush of current at the beginning of the opening of the shutter and immediately thereafter will provide a brief rush of counter-current to brake the opening of the shutter.

Other and incidental objects of the invention will be apparent to those skilled in the art from a reading of the following specification and an inspection of the accompanying drawings, in which Figure l is a curve showing the opening or building up time of the usual amplifiersfor different modulation percentages, i

Figure 2 is acurve illustrating the increase of the opening speed of the shutter as accomplished by the aid of an additional current rush,

Figure 3 is a curve illustrating the conditions when after the first current rush an opposite current pulse is supplied,

Figures 4, 5 and 6 are schematic diagrams of circuits for providing an initial current surge as illustrated in Fig, 2, and

Figure 7 is a schematic diagram of a circuit designed to supply not only the initial surge of current but also a subsequent counter-pulse of current as illustrated in Fig. 3.

Fig. 1 shows graphically by curve A the building-up period of a noiseless biasing gate connect ed and operated in the customary way, for different modulation percentages. The gate for its drive is furnished only with the normal noiseless or noise-reduction current. This graph demonstrates that in the presence of percent modulation, around 21 milliseconds are required until the motion has been fully built up. Graph B shows the building-up period of a biasing gate, mask or shutter which is connected and operated according to this invention. Accordingly, at 100 percent modulation, about 4.5 milliseconds are required for building up the motion. This is a period considerably less than before. Curve A moreover shows another disadvantage, namely,

that after the maximum deflection has been attained, the gate keeps on oscillating a few times before a steady position has been attained. Graph B shows that after the crest of the gate motion has been reached, no further oscillation of the gate occurs.

Referring to Fig. 2, curve C represents the distance traveled by or the stroke of the gate. Curve E represents the normal gate current, while curve D represents the additional current supplied to the gate in the form of a pulse or rush. If the gate were fed only with gate current E, the building-up period would be as shown by graph A, Fig. 1. Now, in order to shorten this building-up period the gate is fed with an additional rush of current D prior to the opening motion. By this pulse or rush of current, the opening speed of the gate is appreciably augmented, in other words, the building-up period is shortened. In order to prevent immediate reclosure of the gate, the gate current by the aid of RC networks possessing a definite time constant is caused to rise in such a way thatthe edge of the gate results in the dot-dash line. Curve Crises appreciably faster than under the normal and customary operating conditions.

Referring now to Fig. 3, reference letters E,

C, D again denote what has previously been explained by reference to Fig. 2. After the first current rush D the gate is fed with a counteracting pulse of current F. This pulse F retards the motion of the gate. While this, on the one hand, prevents oscillation of the gate, it is possible, on the other hand, to raise the opening speed of the gate by application of a more powerful initial pulse of current than indicated in Fig. 2, without any risk of the gate acquiring excessive speed and deflecting away beyond the maximum with the risk of its becoming damaged.

Fig. 4 illustrates a circuit organization by which the gate may be supplied with an additional pulse of current in addition to the normal gate current, this pulse being applied thereto prior to the opening. The tonal frequency is fed to the rectifier tube 2 by Way of a transformer I. The tonal frequency or audio frequencywhich is rectified in the said tube 2 is normally supplied to the biasing gate, optionally by way of an amplifier, in fact, such an amplifier is provided according to the invention. The amplifier is indicated at 5. Rectifier and amplifier are coupled by an RC network 3. In the plate circuit of the amplifier tube the gate is connected to the terminals I and II. The amplifier tube is fed with potential by way of a choke-coil 9 and stabilizer means Ii] (indicated by dash-lines) say, from a supply-line, though no details on this point are indicated in the drawings. In parallel relation to tube 5 is a condenser 8. According to the invention the rectifier tube is in direct coupling relation with the grid of the amplifier tube 5 throughthe condenser 4. Further, according to the invention, a resistance 6 with shunted condenser 1 is included in the cathode lead of the amplifier tube 5. The said condenser 4 is intended to make conditions so that the gate in addition to the normal gate current is fed with a supplementary or boosting rush or pulse of current prior to the opening.

The frequency fed to the transformer I creates at the cathode of tube 2 a potential which is directly fed through the condenser 4 to the grid of the amplifier tube 5. The discharge of the condenser 4 is efiected through the RC network 3. The capacity of this network in the mean- .while has also been charged by the potential furnished from tube 2 and thus reaches also the grid of the amplifier tube 5 though later than that of the condenser 4. The first current pulse, as will thus be seen, is controlled by the potential impressed through the condenser 4 upon the grid of the amplifier tube. In order that the initial current pulse fed to the gate may be made as high as feasible the cathode resistance 6 is shortcircuited for brief current pulses by means of a condenser I so that the tube operates practically on the static characteristic and with maximum amplification. For the frequency of the slow charging of the condensers of the RC net- Work the condenser 4 is made very large, with the result that in that case the tubes work with variable grid biasing potential at 6, and that the biasing potential declines accordingly.

As shown in the circuit organization Fig. 5, the initial current pulse could be produced by way of another rectifier tube 2a rather than by the condenser 4, if desired, or by a Westector or copper oxide type of rectifier known in the art. As to the rest the circuit organization may be completed in a similar way as in Fig, 4.

Inasmuch as the cathode resistance means a loss so far as the amplification is concerned, in

Fig. 6, a shunt resistance is provided which cornprises the resistances II and I2 and the condenser I. This shunt may be made variable and may thereby cause changes of the shunt current in the cathode resistance and thus of the grid biasing voltage which may arise, and as a result the amplification. The said shunt resistance here takes the place of the input potentiometer heretofore customarily provided with gates or shutters, and it offers the advantage that the duration of the building-up period of motion is not affected. The shunt resistance, of course, is not confined to the particular type shown in Fig. 6 and may be modified in any convenient way. Fig. 3 illustrates the situation if after the first current pulse has been produced the shutter is furnished with an oppositely acting surge of current. Fig. 7 illustrates a circuit organization which is suited therefor. In this scheme the rectified current isalso furnished to an amplifier. The latter consists of the two tubes [2 and It, the plate of tube l3 being in coupling relation through a condenser 14 with the tube l2. The gate is connected at I and II. In this circuit arrangement the first pulse of current is insured by the condenser 4, while the second pulse which has the opposite action is created by the combined amplifier organization comprising the two tubes l2 and I3. These two tubes i2 and [3 may conveniently be confined inside one bulb in the form of what is known as a twin triode. By means of the potentiometers 6 and 6a the gate current can be so adjusted that in spite of an existing maximum initial speed no residual current will flow in the gate or shutter winding. In other words, there is a chance to furnish the gate with mechanical zero point adjustment, a task that is impossible in the case of gates with a back current.

As already pointed out above, the circuit organization here disclosed insures appreciable abbreviation of the building-up period of gate motion. As has been ascertained by measurements the building-up period in the case of gates connected as hereinbefore described, amounts to 2 milliseconds in the presence of 10 percent and around 4 milliseconds at percent.

It is, of course, evident that these figures by no means limit the building-up periods attainable with the circuit organization here disclosed. Moreover, the invention is not restricted to gates or masking shutters or diaphragms, for it will be found useful wherever the point and desideratum is to abbreviate the building-up period in the case of elements controlled and actuated by electrical current. Thus, for instance, the shift of the zero line could be altered according to the basic idea of the invention.

This type of amplifier is, of course, not limited to use in controlling the ground noise reduction shutters of sound recording apparatus but is equally applicable to the control of any other apparatus requiring a peak reading amplifier as, for example, in the control of volume expanders or compressors or similar apparatus.

Having now described my invention, I claim:

1. An amplifier of the class described including a rectifier, a thermionic amplifier, filter means between said rectifier and said amplifier, and means connected from said rectifier to the grid of said amplifier for transmitting an initial impulse whereby an abrupt transitory change in the output of said amplifier is produced.

2. Apparatus as defined in claim 1 having a aesaoos shunt resistance connected across the plate and cathode of the amplifier tube.

3. Apparatus as defined in claim 1 wherein the means connected from the rectifier to the grid of the amplifier is a condenser.

4. An amplifier of the class described including a rectifier, a filter, an amplifier tube connected to the output of the rectifier, a condenser connected from said rectifier to the grid of said tube, and a second amplifier tube controlled from said first amplifier through a delay network and connected to the output in opposition to said first amplifier.

5. An amplifier of the class described including a rectifier, a filter, an amplifier tube connected to the output of the rectifier, a condenser con nected fromsaid rectifier to the grid of said tube, and a second amplifier tube capacitatively coupled to the first amplifier through a delay network and connected to the output in opposition to said first amplifier.

6. An amplifier of the class described includ ing a rectifier, a filter, an amplifier tube connected to the output of the rectifier, a condenser connected from said rectifier to the grid of said tube, and a second amplifier tube capacitativel'y coupled to the first amplifier through a delay network and connected to the output in opposition to said first amplifier, whereby said first amplifier produces a sudden pulsation of current and thereafter a steady current while said second amplifier produces only a pulsation of current after the pulsation produced by the first amplifier and in opposition thereto.

ERNST HACK. 

